**Experimental Description:**
**Experimental Preparation:**
The experiment used two different sizes of glass beads—small beads with a diameter of micrometers (µm) and large beads with a diameter of millimeters (mm). Tap water and copper sulfate pentahydrate were also used as experimental materials.
**Experimental Equipment:**
A core nuclear magnetic resonance (NMR) imaging analyzer was utilized, operating at a frequency of approximately 22 MHz. The probe coil had a diameter of 15 mm, and the experimental temperature was maintained at 31.99–32.00°C.
**Porosity and Permeability Measurement:**
Porosity and permeability were measured using NMR core analysis software. The system allowed for detailed characterization of the pore structure within the bead samples.
**Magnetic Resonance Imaging (MRI):**
The spin echo sequence was employed to generate T2-weighted images. This provided insights into the distribution of fluids within the porous media.
**Analysis and Results:**
**Porosity Determination:**
Due to the different pore sizes formed by the small and large glass beads, the relaxation times of the water inside these pores varied after saturation. Calibration was performed using pure glass for large beads and saturated small beads for calibration. The calibration data showed distinct differences between the two bead types.
**Saturation and Permeability Measurement:**
Water was gradually added to dry glass beads until full saturation was achieved. Three transient time points were recorded to calculate porosity using the formula:
**Saturation = (Transient Porosity / Saturated Porosity) × (Transient Total Volume / Total Saturated Water Volume)**.
Permeability was calculated using the SDR model with a parameter **a = 1**. The results demonstrated that permeability increased with water content, and at the same saturation level, large beads exhibited higher permeability than small ones.
**Magnetic Resonance Imaging Results:**
Images of small glass beads showed a clear distinction between fully saturated (100% moisture) and partially saturated (14.31%) samples. The brighter region represented water, while the darker area was the solid bead. In contrast, the image of large beads showed changes after agitation, indicating dynamic fluid movement within the sample.
**Pore Size Distribution:**
By analyzing the T2 relaxation time distribution, it was possible to estimate the pore size distribution in both small and large glass beads. As saturation increased, signal intensity rose, but the relative peak areas remained consistent, suggesting that pore size distribution was largely independent of saturation levels.
**Nuclear Magnetic Resonance (NMR) Knowledge:**
NMR has become an essential tool in oil exploration, offering precise analysis of rock pore structures. It provides comprehensive information such as porosity, pore size distribution, permeability, and fluid saturation. Unlike traditional methods like neutron, density, or acoustic logging, NMR can provide three unique types of information:
1. The amount of fluid present in the pores.
2. The properties of the fluids.
3. The size of the pores.
In low-field NMR studies, the focus is on the relaxation behavior of materials, typically expressed through relaxation times. Shorter relaxation times indicate faster signal decay. In porous cores, three main relaxation mechanisms are observed:
1. **Bulk Relaxation:** Occurs in larger pores (>50 nm), determined by the liquid’s physical properties.
2. **Surface Relaxation:** Influenced by pore size, occurring at the liquid-solid interface, with smaller pores showing faster relaxation.
3. **Diffusion Relaxation:** Related to the self-diffusion of liquids, affecting only transverse relaxation.
This experiment highlights the power of NMR in characterizing complex porous systems and understanding fluid dynamics within them.
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